1. Field of the Invention
The present invention relates generally to an image sensing apparatus, and more particularly, to an improvement of an image sensing apparatus such as a video camera having a function of automatically matching focus and exposure in response to a video signal obtained from an image sensor.
2. Description of the Prior Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as a video camera, an approach utilizing a video signal itself obtained from an image sensor for evaluating a state in which the focus is controlled has been developed. According to such an approach, a lot of good characteristics can be obtained. For example, there exists substantially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be precisely matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
As one example of such a focus control method utilizing a video signal, a control method referred to as a so-called hill-climbing servo system has been conventionally known. Briefly stated, in this hill-climbing servo system, digital data obtained by A-D converting a level of a high frequency component of a video signal obtained from an image sensor within the range of a focusing area set in the center of a picture is accumulated every one field using an accumulating circuit. The obtained digital data corresponding to one field is held in a memory as a focus evaluating value, the focus evaluating value is always compared with a focus evaluating value detected one field before, and the relative position of a focusing lens and an image sensor continues to be slightly vibrated such that the focus evaluating value always takes the maximal value. An automatic focusing apparatus which performs hill-climbing control by setting a sampling area in the center of a picture is disclosed in, for example, U.S. Pat. No. 4,614,975.
On the other hand, as automatic stop-down control of an image sensing apparatus such as a video camera, so-called automatic iris control, servo control is performed using as an exposure evaluating value the average value or the peak value of a level of a luminance signal of a video signal obtained from an image sensor such that the illuminance of a picture becomes constant. In such automatic iris control, considering a case in which a video signal is detected over the entire picture and used as an exposure evaluating value, if a bright object such as the sky exists in the picture, the amount of light is lacking with respect to an object in an area other than the sky, so that the picture becomes so dark to be hard to see. As methods for solving such a problem, there have been proposed a system of extracting only a signal in the central portion of a picture using a wide blanking pulse and a system in which a part of a picture is weighed, as compared with the other area, as an area for measuring light. Particularly, Japanese Patent Laying-Open Gazette No. 296673/1987 discloses a system of setting a detection area in a particular position of a picture.
Meanwhile, in the above described conventional automatic focusing apparatus, automatic focusing control is performed such that a focus evaluating value obtained from a level of a high frequency component of a video signal always becomes the maximum. The absolute value of the level of the high frequency component of the video signal changes depending on the degree of defocusing from the in-focus position, and changes depending on the type and the brightness of an object.
Thus, considering a case in which the distance between a lens and an object does not change, when the object moves outside the range of a focusing area set in the picture, or when the brightness of the object changes due to the change in ambient light, a focus evaluating value changes, whereby an automatic focusing operation is erroneously started. If such a malfunction of automatic focusing control occurs, the picture becomes unclear due to unnecessary movement of the lens.
In order to solve such a problem, a system has been proposed which separately determines the change in object by detecting a value of the aperture of a lens and prevents a malfunction of automatic focusing by disabling an automatic focusing operation in response to the result of determination, which is disclosed in U.S. Pat. No. 4,531,158.
However, even in this method, it is not necessarily possible to obtain a good picture by preventing a malfunction. More specifically, if a sampling area for automatic focusing and a sampling area for automatic iris do not coincide with each other, it is impossible to obtain exposure most suitable for an object simultaneously with focusing of the object, so that automatic focusing control is unnecessarily affected due to the change in brightness of an area other than the object.